Improved Ozone Bleaching by Impregntion of Chemical Pulps with an Acidified Mixture of 1,4 Dioxane and Water, 1996 International Pulp Bleaching Conference Proceedings

Ozone has become one of the alternative pulp bleaching chemicals to chlorine and chlorine dioxide in the Pulp and Paper Industry. However, commercial implementation of ozone pulp bleaching is still limited by the severe cellulose degradation which occurs at higher ozone charges. In this paper, a patented technique which improves the lignin-cellulose selectivity during ozone bleaching to a level comparable to that of chlorine based bleaching will be described. The technique is based on the finding that 1,4- dioxane, when present in the right proportion with water inside the pulp fibers, serves as an effective cellulose protector during ozone bleaching. When hemlock kraft pulp with a kappa number of 31.9 and viscosity of 35.8 mPa.s was impregnated with a 70% dioxane solution of pH 2.3, and subsequently was treated at about 40% consistency with a 2% ozone charge, a semi-bleached pulp with a kappa number of 4 - 5 and viscosity of about 28 mPa.s can be obtained after a standard oxidative extraction. This result shows a lignin -carbohydrate selectivity similar to that of chlorine dioxide bleaching. It is found that the presence of dioxane in the impregnation liquor does not affect the delignification efficiency. The effects of process parameters, such as pH, dioxane concentration and ozone charge, will be described. In the remaining part of the paper, the underlying mechanism of the cellulose protection during this novel ozone bleaching technology is examined. It is proposed that dioxane may scavenge the hydroxyl radicals generated in the ozone-lignin reactions. In addition, the dioxane containing impregnation liquor may deswell the carbohydrate-rich fractions while swell the lignin-rich fractions so that the accessibility of the carbohydrate-rich fractions relative to that of the lignin-rich fractions by ozone becomes smaller in dioxane containing solution than that in water.